The present invention relates to the art of cathodochromic cathode-ray tubes, and more particularly, to an improved method for erasing the target in the tube.
In the field of communications, a new generation of teleconferencing systems is being developed. The heart of these systems is a cathodochromic cathode-ray tube (CCRT) that provides for the first time the answer to the need for an economical, as well as a highly efficient projection display device. With the advent of this system, teleconferencing with a large number of terminals exhibiting high efficiency images is becoming economically feasible.
As explained in the prior art references covering the CCRT projection system, including Todd's prior U.S. Pat. No. 3,968,394, issued July 6, 1976, an electron beam is utilized to generate an image on a phosphor coated target panel inside the tube. A high intensity light source illuminates the panel and the image is reflected by a Schmidt mirror and then projected through a lens unit onto a viewing screen.
One essential feature of making the CCRT system feasible is the ability to efficiently erase the image from the target so that the next image can be written by the electron beam. The erasure procedure must be done quickly in order to allow optimum use of the system. It must also be performed so as to assure a substantially white phosphor surface in order to provide for the desired maximum contrast for the next image. Furthermore, the erasure procedure must not damage the phosphor material and /or the tube components.
The prior art erasure procedure that was developed, as shown graphically in FIG. 1 of the drawings, has been successful. The procedure is one of several key inventions that have allowed the commercialization of the CCRT system.
In the prior procedure or method, the first step is to project a defocused electron beam onto the target, as represented by the shaded target portion in FIG. 1. Next, the beam is focused to raise the energy level, as represented by the heavily shaded area of the target in step 2, but establish the energy level below the erase threshold.
The third step of the prior method requires focusing the electron beam tighter so as to actually cause erasure on the target (as depicted by the blank area of the scan line in step 3.) Because of the scattering of the heat energy, a characteristic color border is formed around the erase line. Typically, the focused erase beam in the prior method is 0.008 inch in width (horizontal thickness) with the color border being approximately 0.001 inch (one mil) wide. In performing step 3, it will be recognized that the erase line requires a relatively high energy density on the target. The step 3 of the procedure is highly tedious since there is a tendency for the erase line to overfocus adjacent the edges of the target. With overfocus causing a change in the width of the scan line by a small amount can result in a considerable increase in the energy. For example, a change in the width by a factor of 2 can result in the increase of the energy density by a factor of 4. With this sharp increase, there is a probability of overheating the phosphor material thereby causing permanent damage to the tube.
In the next step, step 4 of the prior art, there is a requirement for setting up multiple erase lines spaced apart. Each of these lines includes the white area with the peripheral color border, each of which is a total of approximately 8 mils in width. With the spaced lines so positioned, any waviness due to electronic noise can be seen more readily. As will be realized, the waves between the scan lines have the potential of forming gaps in the scan pattern in which case there are spots on the target where the phosphor material is not heated sufficiently for erasure. Thus, when the next image is written and projected, the foreign spot is included. The end result is that the tube must be taken out of service and the set up repeated in an attempt to adjust and remove the wavy portions of the adjacent lines. As will be realized, this step, as well as step 3, is highly tedious and time consuming.
In the fifth step of the prior art, the multiple erase lines are squeezed together until they overlap. The adjacent color borders will merge providing increased heat energy thus effectively causing erasure of the phosphor material at the overlapped borders. During this step 5, any further adjustment of the focus end energy levels is made in order to form a composite erase area. When the erasure is assured, the full target is scanned forming under ideal conditions a full erased, white target with a peripheral color border, as shown in step 5 of FIG. 1.
To our knowledge, there have been no other successful methods of CCRT erasure techniques. Thus, while the prior technique or method developed by Todd is successful, there is a need for improvement. Not only is there a need for a more reliable technique to assure full erasure (while at the same time preventing damage to the phosphor material), but also there is a need for providing a simpler and more reliable related tube set up method.